Anti-lock braking systems have now progressed to the point where they are standard on many vehicles. The use of traction control systems is now becoming increasingly widespread, and it is anticipated that their use will parallel that of anti-lock braking systems. In both systems, which may be termed "vehicle control systems," rapid deployment of brake calipers or brake shoes are necessary in order to perform the intended control function. In anti-lock braking systems, when locking of the wheels due to over-application of brake pressure or loss of traction due to the nature of the surface, i.e., gravel, ice, or snow, is encountered, the automotive braking system rapidly pulsates the brakes between an off and an on condition, allowing maximal retention of braking ability while yet retaining the ability to steer the vehicle stably. In traction control systems, loss of traction in a driving wheel is countered by a momentary application of brake pressure, thus restoring traction. In either case, high pressure systems are desirable to effect the rapid changes necessary to achieve the desired control.
In general, the pressure generated by the brake master cylinder cannot be relied upon to achieve the desired results, as response time in such systems is marginal. Moreover, the rapid cycling of the brakes during ABS would quickly deplete the fluid in the master cylinder. To overcome these drawbacks, high pressure pumps, eccentrically driven by an electric motor, supply the high pressure needed to actuate the system. Driving the high pressure pump at all times would be wasteful of energy and further create unwanted noise. Thus, the motor-driven high pressure pump is actuated only when the need for high pressure is sensed by the circuitry associated with the anti-lock braking system or traction control system, as the case may be.
Due to the fact that the high pressure pump is not continually driven, a time lag may exist between the time the motor-driven high pressure pump is actuated and the pump is able to deliver high pressure fluid. Under certain circumstances, the pump may lose its prime, thus further delaying the fluid delivery. It is also desirable to isolate the master cylinder from pressure pulse feedback which can be sensed by the operator. To overcome the aforementioned problems, high pressure accumulators have been used. Such accumulators are filled with high pressure fluid by driving a piston against the force of a strong return spring when the system is initially turned on, for example when the vehicle is initially started. As a result, not only is response time decreased, but the master cylinder may now be isolated during ABS, reducing brake pedal feedback.
If the high pressure pump continues to output high pressure fluid even for a short period of time after the system has become filled, without providing an opportunity to allow fluid to escape to a lower pressure reservoir, both the pump, pressure supply lines, and other components may be damaged by the high pressure. To overcome this problem, bypass valves which operate when a given pressure is reached have been devised. More recently, the high pressure bypass valve and high pressure accumulator have been combined into a single unit, thus saving space as well as number of components.
Combination high pressure accumulator/bypass valves contain sealing rings designed to eliminate the leakage of high pressure fluid. In the past, these seals have been located on the piston element, which slides in a bore in the accumulator body which itself generally shares the same body or housing as the high pressure pump. This same body often also contains sole-noid-actuated control valves and other components necessary for the ABS or TCS system. Thus, to minimize weight, it is desirable to make this body out of a light alloy material such as aluminum. Unfortunately, the combination of high pressure being applied against the sliding seal which, in turn, bears against the light alloy pump body, may induce wear due to the abradability of the light alloy.
Moreover, high pressure accumulator/bypass valve designs of the past have utilized a large number of individual components, including numerous sealing rings, which drive up the costs of both manufacture and assembly.